1. Field of the Invention:
The present invention generally relates to nuclear fuel assemblies for use in nuclear reactors, and more particularly, to a new and improved nuclear fuel assembly especially adapted for use in a boiling water reactor (BWR) to improve the undermoderated condition which typically exists at the top of BWR fuel assemblies.
2. Description of the Prior Art:
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of nuclear fuel rods are grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical nuclear fuel assembly may be formed by a 7.times.7 or an 8.times.8 array of spaced apart, elongated rods supported between upper and lower tie plates. Examples of such typical nuclear fuel assemblies are depicted and described in U.S. Letters Pat. Nos. 3,350,275; 3,466,226; and 3,802,995. In a typical boiling water reactor a nuclear fuel assembly having an 8.times.8 array, the sixty four rods that form the array may be either sixty four fuel rods or may contain one or more non fueled, water moderator rods with remaining rods being fuel rods. A common problem in typical boiling water reactor nuclear fuel assemblies of the types depicted in the above identified patents is that the central region of such fuel assemblies may be under moderated and over enriched. In order to increase the flow of moderator, one or more of the above-mentioned elongated, water moderator rods may be substituted for fuel rods in the central region of such nuclear fuel assemblies. For example, water moderator rods 41 and 42 are depicted in and described in the above identified '995 Patent. However, the use of such moderator rods significantly reduces the amount of fuel in the core since each moderator rod deplaces a fuel rod in the core lattice and contains no fuel.
Other fuel assembly designs, such as that depicted and described in commonly assigned, copending U.S. patent application Ser. No. 368,555 filed on Apr. 15, 1982 utilizes an elongated flow channel which is divided into four separate fuel sections by a central water cross. In the water cross type fuel assembly, each fuel section is provided with an elongated fuel subassembly or mini-bundle, particularly a 4.times.4 fuel rod array.
Both the water tubes of the 8.times.8 array or the water cross of the mini-bundle type of fuel assembly conducts subcooled water from the bottom of the reactor core to the top of reactor core. While the top of the core requires such additional moderator, the bottom of the core is already adequately moderated and does not require such additional moderation.
A typical BWR fuel assembly, such as those discussed above, are highly heterogeneous. The void content or steam fraction in the core varies axially from zero percent at the bottom of the core to about seventy percent at the top of the core. The average void content at the center of the core being from about twenty to forty percent. It is for this reason that the core is over moderated at the bottom and highly under moderated at the top. The undermoderation at the top means that an unfavorable hydrogen to uranium ratio (H/U) exists which results in less burnup and therefore a fuel cycle cost penalty. This is true even where a BWR lattice has been optimized with regard to the H/U ratio since such optimization improves the moderation at an average void content only and the penalty due to the high void content at the top half of the fuel assembly is still present.
Fisher et al, in U.S. Letters Pat. No. 3,799,839 discloses an arrangement for providing a stationary power distribution in the core which involves introducing spatially distributed burnable poison into fuel lattice which is intended to match the exposure dependent control requirements of the core throughout the operating cycle in order to minimize peaking factors.
However, a need still exists to improve the H/U ratio at the top of the fuel assembly without introducing additional moderation at the bottom of the fuel assembly.